The endodontic treatment of a root canal includes the preparation of the root canal by extracting any soft dental tissue and shaping of the root canal followed by the obturating of the root canal with a suitable filling material. Typically, a procedure consists of opening the crown with a series of burs and diamond cutting instruments. Once the crown has been accessed, the pulp is removed with endodontic instruments, leaving the root canal space empty. This space is then disinfected. Subsequently, the canal space is filled with an inert material such as gutta percha and then the crown of the tooth is restored which may involve replacing the natural crown with a crown made of metal or porcelain.
Due to the small dimensions of the root canal, endodontic instruments, such as files and reamers, generally need to be small in size and made of hard materials to cut the dentin. In view of these constraints and the limited elasticity of the material of the endodontic instrument, a risk of the failure of the endodontic instrument exists during the treatment.
The preparation of the root canal by using rotary endodontic files driven by dental handpieces is known. Dental handpieces are able to apply a high torque to the endodontic file which is sufficient to break the endodontic file in the root canal. In this catastrophic case, a portion of the endodontic file remains in the root canal. As a consequence, time consuming and additional treatment steps are required which are uncomfortable for the patient, or the tooth to be treated may even have to be extracted.
From the prior art, a number of measures are known for avoiding that a rotary endodontic file breaks off in the root canal during the procedure.
In general, such measures lead to a complication of the preparation of the root canal and thereby cannot be considered to be desirable in the context of endodontic treatment.
Specifically, endodontic file systems including a set of files are used in sequence, for example in the “crown down” technique or “step back” technique. In the “crown down” technique, instruments with larger tapers are followed by instruments with smaller tapers in order to cut dentine from the crown to the apex and not along either the full inner surface length of the root canal or along the length of the file surface.
Accordingly, by using rotary endodontic files, it is impossible to prepare the entire root canal with a single rotary endodontic file without accepting a high risk that the file breaks off in the root canal during the procedure.
As an alternative to the endodontic treatment of a root canal by using rotary endodontic files, dental handpieces were suggested which alternate the direction of rotation of the rotary endodontic file at a frequency of from 1 to 20 Hz between a first direction and a second direction opposite to the first direction (reciprocating). The first direction may be the cutting direction of the endodontic file wherein the arc of rotation is larger than the arc of rotation in the second direction which is opposite to the first direction.
Given that the endodontic file changes the direction of rotation periodically, the risk of the file breaking off in the root canal is reduced provided the dental handpiece does not exert a torque in the order of the elastic limit of the endodontic file while the endodontic file is immobilized in the root canal.
However, even if the endodontic file gets immobilized in the root canal during rotation in a first direction which is usually a cutting direction, it is possible that the subsequent rotation in the opposite direction liberates the file so that the preparation of the root canal in endodontic treatment is not interrupted.
EP 10 013 364.4 of the present applicant discloses a dental handpiece which allows the preparation of a root canal by using a single file in a reciprocating mode.
However, since root canal instruments operated in the reciprocating mode cut on their full length or at least on more blades or surface area than if operated in continuous rotary mode or in the endodontic “crown down” or “step back” technique, a higher torque load especially in long narrow root canals must be applied when using reciprocating endodontic files.
Accordingly, in case of a reciprocating rotation of the endodontic file, it is possible that the endodontic file gets stuck in the root canal for the following reason:
In practice, a dental handpiece provides a maximum torque of a given magnitude due to the constraints on the dental handpiece regarding acceptable size and weight. In case the maximum torque is applied to the endodontic file in the first direction being the cutting direction, the endodontic file will be worked into the wall of the root canal with maximum torque and then stops. Since the sliding friction coefficient μs is smaller than the kinetic friction μk coefficient, the maximum torque of the handpiece might not be sufficient to liberate the endodontic file when the direction of rotation is changed. Accordingly, despite the reciprocating rotation of the endodontic file, the endodontic file may get immobilized in the root canal.
In order to address this problem, it was suggested to limit the torque applied to the endodontic instrument. For this purpose, it was suggested that a torque threshold should be introduced which limits the torque applied to the endodontic instrument. However, torque control with a reciprocating endodontic file is a complex problem. Different from the situation with a rotary endodontic file driven in continuous rotary mode, the torque applied to the endodontic file operated in reciprocating mode is not proportional to the current absorbed by the electric motor throughout the entire sequence of rotation. The time dependent alternation of the direction of rotation gives rise to time dependent effects on the current absorbed by the electric motor. For example, effects due to the acceleration and deceleration of the endodontic file, limited possibilities of averaging over a time interval, reaction times during the measurement, or effects due to the reversal of magnetisation provide erroneous data which cannot be used for deriving a realistic torque indicative of the state of the endodontic file.
US2002/0064756 discloses a dental handpiece for forming root canals comprising a motor, a cutting tool driven by said motor, and control means for automatically and periodically reversing the motor according to preset rotation periods of the tool in one direction and in the opposite one, respectively. Means for detecting the load torque applied to the tool may also be enabled during continuous rotation, so that the control means reverse the motor when the torque detected by the load torque detection means reaches a preset reference value during continuous rotation.
According to US2002/0064756, besides providing the reversal of motor when the load torque applied to the tool has reached a preset reference torque, the handpiece may use a different control procedure, not making use of the detection of the load torque applied to the tool. In such procedure, after disabling the torque detection means, motor and consequently tool operate in a reciprocating manner, with preset rotation periods TF and TR in the two directions, respectively. Accordingly, US200210064756 does not disclose a method wherein torque control is carried out during reciprocating of the tool. US200210064756 states specifically that during the F+R control, neither the load torque nor the speed are detected. US2002/0064756 recognizes that as to the speed, the motor is reversed at a very fast pace, and therefore is always in a transient state, with operating features which are very far from the standard ones and thus are not considered to be significant. Besides, in the F+R operation, tool is considered not to be subject to any torsion because it is assumed that the reversal of the motion occurs well before that the same tool may be jammed in the dental root canal.
However, the assumptions made according to US2002/0064756 are not correct since despite the reciprocating movement the failure of the tool occurs according to US2002/0064756.